The present invention relates generally to an apparatus for joining sheet material and specifically to a punch and a joint, each having a step therein.
It is common within the metal forming industry to join pieces of sheet metal by punching or otherwise deforming them to cause an interlocking relationship in a localized area. However, these traditional joints have typically required shearing of the sheet material. Thus, these joints tend to leak and also have their corrosion resistant coatings destroyed.
More recently, an apparatus has been used for joining two or more sheets of material together by creating a leakproof and secure joint. These improved conventional joints are created by use of a punch acting against an anvil to produce what is known as a TOG-L-LOC.RTM. joint therebetween. Such a leakproof joint is disclosed in U.S. Pat. Nos. 5,267,383 and 5,177,861, both of which are entitled "Apparatus for Joining Sheet Material" and issued to Sawdon. The disclosures of these patents are incorporated by reference herewithin.
The conventional TOG-L-LOC.RTM. leakproof joints consist of two or more sheets of material having a button or joint formed therebetween by a uniformly cylindrical punch forcibly pushing a punch side sheet of material into interlocking engagement with a die side sheet of material. These conventional leakproof joints have seen tremendous commercial success for use in varied applications such as steel microwave ovens and aluminum automotive bodies. While these leakproof joints have proven reliable and inexpensive, it would be desirable to have an even stronger leakproof joint.
In accordance with the present invention, an improved apparatus for joining sheet material provides a stepped punch. This stepped punch serves to strengthen a joint between two or more sheets of material by creating a stepped segment in at least one of these sheets of material. During formation of this stepped segment, material is forcibly displaced toward another segment of material that is outwardly expanded to interlock with the adjacent sheet of material.
The punch and joint of the present invention are advantageous over conventional punches and joints by achieving a surprisingly stronger leakproof joint. Furthermore, the joint of the present invention has improved wall thickness that is more resistant to fracture as compared to conventional leakproof joints. The punch, joint and method of the present invention thus reduce scrap during the joint forming process. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.